Turns out, ancient sea walls were so study it even baffled late Roman thinkers — and U.S. scientists have discovered the secret to their success is in the salty air and water

Undated handout photo issued by the British Musuem Thursday Sept. 20, 2012 of Pompeii, Bay of Naples, Italy. New science suggests ancient cities sea walls withstood the test of time because their chemical makeup continued to react with salt in the water and air.The Trustees of the British Museum/ AP

It is a mystery that has baffled modern engineers: why do 2,000-year-old Roman sea walls survive to this day, yet modern concrete versions, embedded with steel, crumble within decades?

Even Pliny the Elder, writing in 79AD, noted that concrete structures in ancient harbours “become a single stone mass, impregnable to the waves, and every day stronger”, despite being battered constantly by seawater.

Now scientists in the US think they have found the answer; when saltwater mixes with the volcanic ash and lime used by the Romans, it leads to the growth of interlocking minerals, which make the concrete virtually impenetrable.

“It is a system that’s contrary to everything one would want in cement-based concrete,” said Marie Jackson, a geology and geophysics research professor at the University of Utah, who led the study. “We’re looking at a system that thrives in open chemical exchange with seawater.”

Roman engineers made concrete by mixing volcanic ash with lime and seawater to make a mortar, and then added chunks of volcanic rock. The combination produces what is called a pozzuolanic reaction, named after the city of Pozzuoli in the Bay of Naples, prompting the formation of crystals in the mixture as it sets. The same reaction happens in nature, and clumps of natural cement can be found in volcanic areas, which is possibly what gave the Romans the idea.

Researchers studied cores of concrete from Roman sea defences and found the minerals had grown into the cracks caused by tidal erosion, proving that the saltwater reaction continues even after concrete has set.

In contrast, most modern concrete is not intended to chemically react, and so does not cause mineralisation when mixed with saltwater.

The exact recipe for Roman concrete has been lost, but the team is now working on a cement that will allow sea defences to last for centuries.